1. Field of the Invention
The present invention relates to image display panels and a process for producing the same, and more particularly, to image display panels, especially screen panels of cathode ray tubes, the front surfaces of which are improved in antistatic properties and if necessary, coated with films best suited for minimizing the reflection of external light, and to a process for producing such panels.
2. Description of Related Art
Transparent base plates made of glass or the like are used for image display panels, as represented by those of cathode ray tubes and liquid crystal display devices. It is desired to inhibit these image display panels from electrification, i.e. being charged with electricity, and from the reflection of external light.
As regards the inhibition of electrification, the wide use of explosion-proof cathode ray tubes in recent years has brought about the uselessness of the front protecting glasses of television receivers or of other display devices and, in consequence, the front parts of cathode ray tubes have become bare. This has caused matters such that a person upon directly touching the front part (panel) of a cathode ray tube, is strongly shocked by high voltage electrostatic charge present on the panel surface. Moreover, the electrified panel surface absorbs atmospheric dirt or dust, which accumulates and fouls the panel surface. This raises the problem of images formed on the panel being hard to see. Taking a cathode ray tube (a color Braun tube or a display Braun tube) as an example, the cause of the electrification will now be explained. As shown in FIG. 4 of the accompanying drawings, a thin uniform aluminum film 4 is vapor-deposited on a phosphor coat 3 laid on the inner wall of the glass panel 7 of the cathode ray tube 1. When a power supply to the cathode ray tube is turned on or off, a high positive-pole voltage is applied to the aluminum film 4 or shut off therefrom. To oppose this high voltage on the inside aluminum film, electric charge develops, that is, electrostatic induction generates electric charge on the outer wall of the panel 7.
In addition, the outer surfaces of the panels (image display panels) of Braun tubes are glassy and hence are liable to strongly reflect external light, making it difficult to read images formed on the panels. Recently, in particular, display devices comprising various cathode ray tubes, besides television receivers, have been used widely as terminals of information machines and apparatus. Therefore the problem of this external-light reflection has become taken up extensively in the field of VDT (visual display terminal). For such reasons, a very strong need for anti-reflection films has been growing.
Thus, demand has become very strong for image display panels provided with the function of inhibiting the reflection of external light, particularly the functions of inhibiting both reflection and electrification.
For inhibiting electrification of the faces of image display panels, there are the method of forming transparent electroconductive coats on the front surfaces of Braun tubes in television receivers and various terminal display devices and grounding these coats; the method (Jap. Pat. Appln. Kokai No. 61-118932) of utilizing airborne moisture, that is the method of leaving small amounts of hydroxy groups in Si-O-Si chains (or net-works) formed from the hydrolysis of alkoxysilane applied on the panel face, the hydroscopic nature of which lowers the electric resistance of the glass surfaces of the panels to levels of 10.sup.9 to 10.sup.10 .OMEGA.; and the method (Japanese Patent Application Kokai (Laid-Open) No. 61-118932) of stopping en route the decomposition of a silicon alkoxide such as ethyl silicate applied on the panel faces, thereby leaving some silanol groups (--Si--OH) in the Si-O-Si siloxane structure.
For the formation of transparent electroconductive films, there are known, for example, the method, as shown in Japanese Utility Patent Publication No. 49-24211, of applying an electroconductor solution by spray coating and burning the coats at 450.degree. C. to form transparent conductive films, the method of forming such films by vacuum deposition or sputtering, and the method, as shown in Japanese Patent Application Kokai Nos. 62-154540 and 62-116436, of forming transparent conductive films or extra fine conductive wires in the form of strips or nets.
For inhibiting the panel faces from reflecting external light, there are known, for example, the method of forming so-called telepanels covered with multilayer anti-reflecting films (AR coats), by vapor deposition and adhering these telepanels on the panel faces and the method of spraying the panel faces with an alcoholic solution of Si(OR.sub.1).sub.4 (R.sub.1 is alkyl), followed by burning to form coats having minute projections consisting of SiO.sub.2 particles.
Further, conventional panels inhibited from the reflection of external light include, for example, glass panels the surfaces of which are etched with silicofluoric acid (H.sub.2 SiF.sub.6) to form projections or depressions of 50 to 30,000 .ANG. height or depth and 100 to 2,000 .ANG. pitch, thereby imparting a reflection inhibiting function (U.S. Pat. No. 2,490,662) and glass panels the surfaces of which are sprayed with an alcoholic solution of alkoxysilane Si(OR.sub.1).sub.4 and then subjected to burning to form SiO.sub.2 coats having fine projections or depressions (Japanese Patent Application Kokai No. 61-118932).
The method of utilizing atmospheric moisture to inhibit the electrification is effective in locations where humidity is relatively high, but exhibits no antistatic effect in locations where humidity is low. In addition, the film fixing temperature cannot be raised over 80.degree. C. because some silanol groups (--Si--OH) must be left in the film constitution (at higher temperatures all the silanol groups convert to form the Si-O-SiO siloxane structure). The coating films formed at such low temperatures exhibit very low strengths and are gradually peeled off by rubbing with a cloth.
Vapor deposition methods are not fitted for mass production, since they generally require large-scale apparatus for vacuum deposition, sputtering, CVD (chemical vapor deposition), or the like and need treatment in vacuo. Moreover, these methods involve significant problems in fabrication cost as well as in increase in throughput. The above-mentioned method of forming transparent conductive films in the form of strips requires more operation steps, requiring high production costs. The formation of extra fine conductor wires in the form of strips has many problems in production cost and in product performance.
According to the method of imparting a reflection inhibiting function by etching, the etching leaves a deposit on the treated surface and the etched surface, damaged chemically, has low abrasion strength, that is, rubbing the etched surface readily removes projections therefrom, reducing the reflection inhibiting effect remarkably.
According to the method of spraying an alcoholic solution of Si(OR.sub.1).sub.4, sprayed liquid particles are deposited more thinly toward the center of the object glass panel face, that is, more thickly toward the periphery. Hence it is difficult to form uniform unevenness throughout the glass surface. This raises the problem of such panels displaying images of low degrees of resolution.
According to the method of spraying an alcoholic solution of Si(OR.sub.1).sub.4 directly against the panel faces of cathode ray tubes, followed by burning to form SiO.sub.2 coats having minute projections or depressions, sufficient anti-glare effect can be obtained and the production costs are low, but the burning for the purpose of fortifying the coats decreases the content of hydroxy groups and hence increases the surface resistivities. Therefore the intended antistatic effect cannot be obtained.
According to the method of adhering telepanels provided with AR coats by vapor deposition, excellent reflection inhibiting efficiency can be achieved, but the antistatic effect cannot be obtained since the AR coats consist of an insulator, and in addition the fabrication cost is high.